The need for a rapidly hardening hydraulic binder for the production of concrete has been known for a long time. Up to now, this need was satisfied either by special quick-setting cements per se or quick-setting cements on the basis of mixed cements with additives. Many of these products are used today in the areas of repair, civil engineering or in the area of interior work, such as, e.g., plaster, mortar or screed systems.
The special quick-setting cements per se can be classified into cements which are produced based on a modified Portland cement, and special cements, the clinker base of which is clearly different from the Portland cement.
In the area of quick-hardening, modified Portland cements can be produced from Portland cement clinkers specially burned for this application case.
In contrast to normal Portland cement clinkers, these clinkers are distinguished by high lime standards—in general >100—and high silicate moduli—in general >3. As a result, the tricalcium silicate (C3S) content, which is essentially responsible for the early strength development of the cement, is markedly increased as compared to normal Portland cements. The cements produced from these clinkers achieve an early strength that is up to 20% higher than that of the normal Portland cements.
The group of modified Portland cements also includes cements which contain no or only very little sulphate carrier (generally calcium sulphate) as setting regulator. If necessary, the C3A content is increased with the respective clinkers. These cements cause the rapid setting of the concrete or mortar and are therefore used predominantly in the area of gunite. Often, non-alkaline accelerators, such as aluminum sulphate or aluminum hydroxide, or alkali-containing accelerators based on alkali hydroxides, alkali liquid glass or other alkali salts are added in order to increase the green strength. Since these cements essentially accelerate only the setting process, but not the actual strength development, they should be only conditionally assigned to the group of quick-setting cements.
Special cements used to accelerate the hardening process differ from the Portland cements by a different composition of the clinker that forms their basis. Typical representatives of this group of cements are aluminous cements, sulphoaluminate cements and regulated set cements.
Aluminous cement or high alumina cement is also often referred to as calcium aluminate cement. The humidity-determining clinker phases are monocalcium aluminate (CA), calcium dialuminate (CA2) and mayenite (C12A7). The silica based clinker phases (C2S and C2AS) that are also present contribute insignificantly to the strength. Aluminous cements can reach extremely high compressive strengths within the shortest amount of time even at low temperatures. Compressive strengths of up to 60 N/mm2 after 24 hours can be attained unerringly with these cements. The two hydrate phases CAH10 and C2AH8 that are formed are responsible for this high early strength. In the course of time, these hydrate phases transform into thermally stable C3AH6 favored by humidity and temperatures of >23° C. The transformation is associated with a strong increase in porosity and release of water and is the cause of a strong reduction of strength. Based on this background information, aluminous cements may not be used for construction engineering buildings in accordance with EN 206-1:2001-07 or DIN 1045-2:2001-07.
Among the areas of application of aluminous cements are, in particular, fireproof materials, well cement formulations and the area of building chemistry.
Calcium sulphoaluminate cements or calcium aluminate sulphate cements contain calcium aluminate sulphate C4A3S, dicalcium silicate (C2S) and calcium aluminate ferrite (C2(A, F)) as main clinker phases. Depending on the raw mix composition, larger quantities of magnesium, fluoride or iron can be integrated into the clinker phases. In the early hydration, large amounts of ettringite and monosulphate are formed, which result in a considerable early strength (at times up to 30 N/mm2 after 3 hours). Due to the volume expansion occurring in the course of hydration, these cements are often used as shrinkage compensators, e.g., in screeds. Structural components made from this cement tend to exhibit rapid carbonation accompanied by considerable loss of strength. Contrary to the uses of Portland cement, the reinforcing steel or prestressing steel in structural parts made from this cement are not protected from corrosion. In general, these cements are only used in blends with other cements.
Regulated set cement or jet cement or calcium aluminate fluoride cement contains primarily the fluoroaluminate-containing phase C11A7.CaF2 and gypsum, in addition to the main clinker phase C3S. Most of the time, these cements also contain a lime component. During hydration, the fluoroaluminate forms large quantities of ettringite and is responsible for the high early strength of up to 16 N/mm2 after one hour. In order to prevent overly rapid setting of the concrete or mortar, setting retarders are often added. This cement was able to hold its ground only in niche products on the market. Structural components made from this cement exhibited poor durability on exterior surfaces.
In addition to the above-named special cements, there are also some early high-strength cements based on alinite (C11A7.CaCl2). However, these cements hardly have a practical significance due to their corrosion-promoting effect with respect to reinforcing steel.
In most cases, more rapidly hardening hydraulic binders are produced as mix cements with various additives. The basis of these systems is always a Portland cement or Portland cement clinker.
The vast majority of these mix cements contains additives which stimulate the aluminatic and/or alumino-ferrite clinker phases and thus accelerate the formation of ettringite, monosulphate or even calcium aluminate hydrate. By the addition of soluble aluminum compounds (e.g., aluminum sulphate, aluminum hydroxide) or an aluminous cement component, the portion of strength-forming hydrate phases can be further increased.
Alkali carbonates, fruit acids or sulfonates are often used to accelerate hardening. In addition, to control the processing properties and strength development characteristics, various additives (e.g., fly ashes, microsilica, metakaolin) and/or admixtures (liquefier, phosphate retarders or the like) are added.
In the following, a number of examples of patented mix cements based on Portland cement are presented.
In EP 0517869 B1, a system based on Portland cement is described. In order to be able to generate high strengths, the portion of C4AF should be greater than 9.5%. A carbonate donor is added to the system as K2CO3, -bicarbonate or -trihydrate as well as tricalcium citrate monohydrate (if necessary blended with dipotassium oxalate monohydrate). Using this system, strengths of up to 20 N/mm2 can be attained after 4 hours.
DE 4223494 C2 describes a system based on Portland cement having a small portion of aluminous cement and additives of sodium carbonate, sodium sulphate, calcium hydroxide, lithium carbonate, Kann tartate and Ca lignine sulfonate. Strengths of up to 7 N/mm2 are achieved after one hour.
In DE 4313148, an accelerated system is described which consists of Portland cement, microsilica or metakaolin, sodium citrate and lignine or naphthaline sulfonate. Strengths of up to 4 N/mm2 are achieved after one hour.
DE 10141864A1 describes a quick-setting cement binder mixture consisting of extremely fine powder of Portland cement clinker, gluconic acid or gluconate, alumina hot melt cement and, if necessary, further additives. The attainable strength can be over 20 N/mm2 after 4 hours.
The acceleration of the systems via the increased formation of calcium aluminate hydrate, ettringite and/or monosulphate accounts for a reduced durability in certain partial areas of the respective concretes. Furthermore, such binders can bring about undesired brown colorations on the concrete surface due to the complexation of iron caused by the alkali carbonates and/or the hydroxycarboxylic acids. And last but not least, a number of substances used to regulate the systems result in an increased hygroscopic moisture absorption of the cement and thus reduce its suitability for storage.
Therefore, these products could not establish themselves in the area of concrete construction engineering. But they are used in the area of repairs and maintenance, in the area of the plaster and mortar industry and in the area of building chemistry.
The accelerated formation of the C—S—H phases that are decisive for the development of strength is far more effective than an acceleration of the aluminate and/or aluminate ferritic reaction, which result in hydrate phases with limited strength potential.
The acceleration of the formation of C—S—H phases can be realized in the mix cement by the use of salts which increase the solubility product with respect to calcium. This includes halogenides, pseudohalogenides, nitrates, nitrides and formiates. The respective calcium salts and salts with other polyvalent cations are especially suited with respect to acceleration. Alkali cations reduce the solubility product with respect to calcium and oppose an accelerated formation of C—S—H phases.
Of the above-mentioned substances, the halogenides have the greatest effect on the formation of the C—S—H phases. Due to the excellent availability and the low cost, calcium chloride was used in the past to accelerate cement hydration. But it turned out that the chlorides and the other halogenides significantly promote the corrosion of pre-stressing steels and reinforcement steels. For this reason, these substances may only be used for constructive steel and prestressed concrete, specifically in accordance with EN 206-1:2007-07. In addition, these accelerators result in markedly reduced final strengths and an insufficient durability of the parts made from them. But the accelerating effect is less developed with the pseudohalogenides, nitrates, nitrides and formiates. However, these salts can also promote the corrosion of the stressed or unstressed steels in the concrete. Their application for steel or prestressed concrete is limited and is subject to the respective national regulations in the area of the CEN states. Thus, for example, in Germany, only formiate may be used to accelerate the hardening process in steel concrete constructions. Its use for prestressed concrete is prohibited.
The use of gypsum as a hardening accelerator is problematic due to the ettringite formation with respect to the usually targeted durability and strength of the concrete under stress.
Another quick-setting, cement-based hydraulic binder exhibiting low shrinkage, in particular for plasters and screeds, is known from DE 197 54 826 A1. In accordance with this publication, the ettringite problem is to be solved by affecting a specific ettringite formation early on by the separate addition of a reactive CaSO4 compound and by the fact that, if possible, no secondary ettringite is formed following the hardening phase.
However, the setting properties of this known binder are not yet satisfactory.
In addition—specifically in terms of the production of concrete building parts, specifically prefabricated concrete parts—no quick set cement is known which allows the completion of an entire concreting process, including rapid form stripping and loading of the concrete building parts within one work shift under normal conditions. Therefore, there is a lack of a hydraulic binder which provides risk-free satisfaction of the requirements of concrete production and/or which has the desired economic properties of the concrete made from the binder.